There’s a fair bit. Especially as you get to a point a refinery needs turn around.
A typical variation will be +/- 5 points on the VI. So like in a group 2, 112-117 we’ll say hypothetically.
When turn around is needing to happen, you hypothetically might see 105-107.
A lot of the variation also happens depending on feed stocks. So GTL is going to be more consistent (by nature of the process.) Where as crude based oils will have more variability depending on the inbound feed stream through the refinery. This is dictated to by price / availability / season for crude. And how the refinery is over all tweaked.
You make this up in blending. As you’re not using just a straight cut of base oil while blending for an PCEO product. Especially a synthetic blend. So, the lab will get a CoA of each tanker load of base oil. They will enter the test results into the system. This will then, tell them what they need to blend to meet their specific requirements. Thus, the end user will see very little variation.
You see this variation more on straight cuts or super heavy blends. So your AW hydraulic oils, R&O oils, gear oils, etc. So you can go from a heavy AW32 to a light AW 32. It’s very noticeable in the heavier grades of product - your EP460 & 680, etc. that may even need to be hit with bright stock to make them heavy enough.